Tutorial: Remove wildcard import of enum variants

[skearya]

In the channels section of the tutorial, a use statement imports all variants of the Command enum,

    while let Some(cmd) = rx.recv().await {
        use Command::*;

        match cmd {
            Get { key } => {
                client.get(&key).await;
            }
            Set { key, val } => {
                client.set(&key, val).await;
            }
        }
    }

This is bad practice because renaming / removing something in the Command enum doesn't cause a compile time error, like in a normal match statement, and creates a catch-all in the match.

Code_WJwujKAkVD.mp4

This PR just removes the use Command::* and changes the match statement to be on the full enum path in channels.md

[Darksonn]

Thanks.